A communication system can be seen as a facility that enables communication sessions between two or more nodes such as fixed or mobile communication devices, access points such as base stations, servers, machine type servers, routers, and so on. A communication system and compatible communicating devices typically operate in accordance with a given standard or specification which sets out what the various entities associated with the system are permitted to do and how that should be achieved. For example, the standards, specifications and related protocols can define the manner how communication devices shall communicate with the access points, how various aspects of the communications shall be implemented and how the devices and functionalities thereof shall be configured.
A user can access the communication system by means of an appropriate communication device. A communication device of a user is often referred to as user equipment (UE) or terminal.
Signals can be carried on wired or wireless carriers. Examples of wireless systems include public land mobile networks (PLMN), satellite based communication systems and different wireless local networks, for example wireless local area networks (WLAN). Wireless systems can be divided into coverage areas referred to as cells, such systems being often referred to as cellular systems. A cell can be provided by a base station, there being various different types of base stations. Different types of cells can provide different features. For example, cells can have different shapes, sizes, functionalities and other characteristics. A cell is typically controlled by a control node.
A communication device is provided with an appropriate signal receiving and transmitting arrangement for enabling communications with other parties. In wireless systems a communication device provides a transceiver station that can communicate with another communication device such as e.g. a base station and/or another user equipment. A communication device such as a user equipment (UE) may access a carrier provided by a base station, and transmit and/or receive on the carrier.
An example of cellular communication systems is an architecture that is being standardized by the 3rd Generation Partnership Project (3GPP). A recent development in this field is often referred to as the long-term evolution (LTE) of the Universal Mobile Telecommunications System (UMTS) radio-access technology. In LTE base stations providing the cells are commonly referred to as enhanced NodeBs (eNB). An eNB can provide coverage for an entire cell or similar radio service area.
Cells can provide different service areas. For example, some cells may provide wide coverage areas while some other cells provide smaller coverage areas. The smaller radio coverage areas can be located wholly or partially within a larger radio coverage area. For example, in LTE a node providing a relatively wide coverage area is referred to as a macro eNode B. Examples of nodes providing smaller cells, or local radio service areas, include femto nodes such as Home eNBs (HeNB), pico nodes such as pico eNodeBs (pico-eNB) and remote radio heads.
A device may communicate with more than one cell. Communications with more than one cell may be provided e.g. to increase performance. A way of providing this could be, for example, based on carrier aggregation (CA). In carrier aggregation a plurality of carriers are aggregated to increase bandwidth. Carrier aggregation comprises aggregating a plurality of component carriers.
LTE-Advanced is an example of a system capable of providing carrier aggregation. In LTE-A two or more component carriers (CCs) can be aggregated in order to support wider transmission bandwidths and/or for spectrum aggregation. Currently it is envisaged that the bandwidths can extend up to 100 MHz. Depending on its capabilities, it is possible to configure a user equipment (UE) to aggregate a different number of component carriers either from the same frequency band or different ones. A primary component carrier can be provided by a primary cell (PCell) whereas further carriers can be provided by at least one secondary cell (SCell). SCells form together with the PCell a set of serving cells. To enable reasonable battery consumption by the user equipment when aggregating carriers, an activation/deactivation mechanism of SCells is supported. When operated to provide CA a user equipment (UE) is configured with a primary cell (PCell). The PCell is used for taking care of security, Non-Access-Stratum (NAS) protocol mobility, and transmission of physical uplink control channel (PUCCH). All other configured CCs are called secondary cells (SCells).
Inter-site carrier aggregation has also been proposed. For example, it has been proposed that smaller cells could be used in conjunction with macro cells. In dual connectivity, a UE is connected to a macro cell and a small cell simultaneously. An aim of dual connectivity is to decrease mobility related signaling load towards the core network as well as to benefit from user throughput gains by the inter-site carrier aggregation for increased transmission bandwidth and scheduling flexibility. In some aspects dual connectivity is rather similar to CA with the macro cell serving as PCell and the small cells being SCells. However, in dual connectivity different eNBs provide the PCell and the sCell(s) as opposed to only one eNB according to e.g. 3GPP LTE Releases 10 and 11.